The use of a metallized plastic strip embedded within currency paper as a security thread for counterfeit deterrence is described within U.S. Pat. Nos. 4,652,015 and 4,761,205. The security thread is virtually undetected under reflected light and legible under transmitted light to verify its presence.
In commercial situations where verification of currency bills is required, the receiver of the currency bill must subject the currency to a relatively intense light source to read the security thread under transmitted light. With large queues of customers at a bank or supermarket, as well as in places of low level illumination such as bars and restaurants it is difficult to visually inspect the corresponding large number of currency bills. It would be advantageous therefore to have some means of automatically determining the presence of the requisite security thread and confirming authenticity to the teller or cashier.
U.S. Pat. No. 3,980,990 entitled "Ferromagnetic Currency Validator" describes a magnetic detection circuit which first submits a proffered currency paper to a magnetic source to magnetize the ferromagnetic ink used with the signature on the portrait surface of the bill.
U.S. Pat. No. 4,524,276 entitled "Apparatus for Detecting a Security Thread Embedded in a Paper-Like Material" describes an infrared radiation source and two infrared radiation detectors used to determine whether or not a security thread is embedded in the paper-like material and also to determine what the detected security material is made of.
Countries outside of the United States that employ plastic or metal security threads embedded in their paper currency, require that the presence of such security threads be ascertained under transmitted light such as described in the aforementioned U.S. Pat. No. 4,524,276. In accordance with the United States requirement that the currency security thread be detected under transmitted light and not seen under reflected light, both reflective and transmissive determinations must be made for complete verification of the currency.
U.S. Pat. No. 4,980,569 describes a security paper verification device wherein optical means are arranged on opposing surfaces of the currency to determine the absence of any device on the surface of the currency paper while detecting the presence of the device within the currency. This is to prevent attaching counterfeit security threads to the outside surface of the currency paper to replicate genuine currency.
U.S. patent application Ser. No. 694,912 filed May 2, 1991 entitled "Currency Verification Device" describes combination of optical means with inductive or capacitive sensors for verifying the presence of the security thread in currency paper.
U.S. patent application Ser. No. 814,824 filed Dec. 31, 1991 entitled "Security Paper Verification Device" describes optical, magnetic and capacitive sensors used in combination to determine currency authenticity. The dark inks and dyes used in printing U.S. federal reserve notes could provide difficult indication of a metallized security thread when such optical sensors are used, per se.
One purpose of the instant invention is to provide an inexpensive optical verification device that is capable of distinguishing between the dark currency ink and dyes and the metallized security thread in U.S. currency paper.